I have a Fastfire III and while it is a great little sight on my 10-22 takedown, I would not expect it to survive the heat of an Ultimak.
I owned a Maadi with an Ultimak, and the Fastfire only allowed you to barely see the front sight. It sits too high, so to answer your question about Cowittness.....No, it won't.
Aimpoint T1 is the best, but I know from experience, the Primary Arms MD-07 clone will work, and survive the heat just fine. Good luck finding one of those though. They have been out of stock since last spring, and they are now projecting a March delivery.